1. Field of the Invention
The invention relates to spring scales and more particularly to spring scales employing an optical readout apparatus for the detection of scale deflection.
2. Brief Description of the Prior Art
In order to provide spring weighing scales at practical costs certain dimensional manufacturing tolerances have been included in spring specifications. These manufacturing tolerances provided scale springs with variations of spring constant, which variations have detracted from the accuracy of weight readings.
For example, springs employed in leaf spring scales such as that illustrated in U.S. Pat. No. 3,807,517 typically included manufacturing tolerances in leaf thickness, width and length. Variations in spring length within tolerance limits were compensated by adjustment at the spring end clamps. Further, length and width variations within tolerance limits were but minimal compared with the length and width dimensions; thus, relatively small errors were encountered due to tolerances in these dimensions. Variations in thickness, however, could not be easily compensated and have produced significant variations in spring constants due to the relatively small thickness dimensions. A typical variation of 1.3% in leaf spring thickness has resulted in a 5% variation in spring constant.
The prior attempt to provide a compensating adjustment at an optical detector for variation in spring constant has not met with commercial success due to numerous disadvantages inherent in the prior optical detectors. U.S. Pat. No. 3,186,148 is illustrative of such prior attempt and discloses a spring scale comprising a pair of vertically spaced, parallel leaf springs, which was utilized in a mail processing system. An optical detector associated with the scale included a bank of multiple photodetectors at the end of a light path. A descending shutter carried by the tare was operable to gradually block the light path to successive photodetectors of the bank. The number of photodetectors remaining in the light path provided a signal indicative of tare deflection, which signal was utilized to set the postage to be imprinted on the piece of mail.
The photodetector bank was mounted to a generally vertical arm which was pivotable to cause the vertical distance between successive photodetectors of the bank to change with respect to the direction of shutter movement. Photodetector arm adjustment effected a compensation for variations in spring constant.
Several disadvantages accompanied the use of this prior optical readout apparatus. For example, numerous photodetectors were required to provide weight indicative signals over a small weight load range. Furthermore, the number of photodetectors increased with increased scale capacity. Additionally, the particular transition point wherein each photodetector switched, for example from conduction to nonconduction state, had to be individually set for successive postage weight increments. These transition points were highly critical, and tolerance requirements rendered final adjustments difficult and time consuming.
A further disadvantage of this prior system related to the relatively large projection path, e.g. 18 inches to 24 inches (45-60 cm.), which was required in order to obtain the necessary optical magnification. As a result of such large projection path and the critical switching points, accuracy of readouts was highly sensitive to vibrations and other environmental conditions.